In lithography techniques, for example, a resist film composed of a resist material is formed on a substrate, and the resist film is subjected to selective exposure of radial rays such as light or electron beam through a mask having a predetermined pattern, followed by development, thereby forming a resist pattern having a predetermined shape on the resist film.
A resist material in which the exposed portions become soluble in a developing solution is called a positive-type, and a resist material in which the exposed portions become insoluble in a developing solution is called a negative-type.
In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have led to rapid progress in the field of pattern miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength (increasing the energy) of the exposure light source. Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers and ArF excimer lasers are starting to be introduced in mass production of the semiconductor elements. Furthermore, research is also being conducted into lithography techniques that use an exposure light source having a wavelength shorter (energy higher) than these excimer lasers, such as electron beam, extreme ultraviolet radiation (EUV), and X ray.
Resist materials for use with these types of exposure light sources require lithography properties such as a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of exposure light sources.
As a resist material that satisfies these conditions, a chemically amplified resist composition is used, which includes a base material component that exhibits a changed solubility in a developing solution under the action of acid and an acid generator component that generates acid upon exposure. For example, in the case where the developing solution is an alkali developing solution (alkali developing process), a chemically amplified positive resist which contains, as a base component (base resin), a resin which exhibits increased solubility in an alkali developing solution under action of acid, and an acid generator is typically used. If the resist film formed using the resist composition is selectively exposed during formation of a resist pattern, then within the exposed portions, acid is generated from the acid generator component, and the action of this acid causes an increase in the polarity of the base resin, making the exposed portions soluble in the alkali developing solution. Thus, by conducting alkali developing, the unexposed portions remain to form a positive resist pattern. On the other hand, in the case where such a resist composition is applied to a solvent developing process using a developing solution containing an organic solvent (organic developing solution), the polarity of the base resin at exposed portions is increased, whereas the solubility at exposed portions in an organic developing solution is relatively decreased. As a result, the unexposed portions of the resist film are dissolved and removed by the organic developing solution, and a negative resist pattern in which the exposed portions are remaining is formed. Such a solvent developing process for forming a negative-tone resist composition is sometimes referred to as “negative-tone developing process” (for example, see Patent Document 1).
In general, the base resin for a chemically amplified resist composition contains a plurality of structural units for improving lithography properties and the like. For example, in the case of a resin composition which exhibits increased solubility in an alkali developing solution by the action of acid, a structural unit containing an acid decomposable group which is decomposed by the action of acid generated from an acid generator component or the like and then exhibits increased polarity. Further, a structural unit containing a lactone-containing cyclic group or a structural unit containing a polar group such as a hydroxy group is used (for example, see Patent Document 2).
Recently, a base resin which has a structural unit containing a cyclic group containing an —SO2— structure has been proposed. It is presumed that such as base resin contributes to improvement in various lithography properties such as mask reproducibility and improvement in shape of resist pattern such as reduction of roughness. The roughness means surface roughness of a resist pattern and causes defects in the shape of a resist pattern. For example, roughness on the line width of a pattern (line width roughness (LWR)) causes various defects such as non-uniformity of the line width of line and space patterns. Such defects of a resist pattern adversely affect the formation of very fine semiconductor elements, and improvement of this characteristic becomes more important as the pattern becomes smaller.
In recently, as the miniaturization of patterns proceeds, a polymeric compound useful as a base resin for a resist composition is demanded.
In Patent Document 3, a resin having an acid generator group which is decomposed upon exposure and then generates acid has been proposed. In the invention of Patent Document 3, as a base resin, a resin obtained by copolymerizing a monomer which derives a structural unit having an acid generator group which generates acid upon exposure, a monomer which derives a structural unit having a cyclic group containing an —SO2— structure, and a monomer which derives a structural unit having an acid decomposable group which is decomposed by the action of acid and then exhibits increased polarity is used.
Such a resin has both the function as an acid generator and the function as a base component, and hence, it can compose a chemically amplified resist composition by itself.